1. Field of the Invention
The present invention relates to a metal oxide film and a method of manufacturing the metal oxide film.
In detail, the present invention relates to a method of manufacturing a metal oxide film that has both of high density and excellent in film strength and is formed on a substrate made of such as glass or plastic, and the metal oxide film obtained by the method of manufacturing the metal oxide film, and further relates to an element using the metal oxide film, a substrate with the metal oxide film, and device using the substrate with the metal oxide film.
2. Description of the Related Art
As a material for forming a transparent conductive film for use in a transparent electrode for a display element such as a liquid-crystal display, electroluminescent display, and a plasma display, in a transparent electrode such as a touch panel, and a solar panel, or for functional coating for such as reflecting heat rays, shielding electromagnetic waves, preventing charging and defogging, a tin-doped indium oxide (Indium Tin Oxide, which may be hereinafter referred to as “ITO”) is known.
As methods of manufacturing this transparent conductive film made of ITO (ITO film), vapor deposition methods (vapor phase methods) are widely used, such as a vacuum deposition method, a sputtering method, and a chemical vapor deposition method. By these methods, a uniform ITO film (a transparent conductive film) being excellent in transparency and conductivity can be formed on a substrate.
However, a film forming apparatus to be used in these methods takes a vacuum container as a base, which is very expensive. Also, the component gas pressure in the manufacturing apparatus is required to be precisely controlled for each substrate film formation, thereby posing a problem in manufacturing cost and mass producibility.
As a manufacturing method to solve these problems, a method has been suggested and studied in which a coating liquid for forming transparent conductive film obtained by dissolving an indium compound and a tin compound in a solvent is used for coating a substrate (this method may be hereinafter referred to as a “coating method” or a “wet coating method”).
In this coating method, an ITO film (a transparent conductive film) is formed with a simple manufacturing process of coating a substrate with the coating liquid for forming transparent conductive film, drying, and heating. Known methods of coating of the substrate with the coating liquid include an inkjet printing method, a screen printing method, a gravure printing method, an offset printing method, a flexor printing method, a dispenser printing method, a slit coating method, a die coating method, a doctor blade coating method, a wire bar coating method, a spin coating method, a dip coating method, and a spray coating method.
As coating liquids for use in these coating methods, various coating liquids containing an indium compound and a tin compound have been conventionally developed. For example, a mixture of indium nitrate and alkyl tin nitrate containing halogen ions or a carboxyl group (for example, refer to Japanese Unexamined Patent Application Publication No. 57-138708, a mixture of an organic indium compound and an organic tin compound containing an alkoxyl group or the like (for example, refer to Japanese Unexamined Patent Application Publication No. 61-26679), a mixture of indium nitrate and an organic tin compound (for example, refer to Japanese Unexamined Patent Application Publication No. 4-255768, an inorganic compound mixture of indium nitrate, tin nitrate, and others (for example, refer to Japanese Unexamined Patent Application Publication No. 57-36714, a mixture of an organic indium nitrate such as dicarboxylate indium nitrate and an organic tin nitrate such as alkyl tin nitrate (for example, refer to Japanese Unexamined Patent Application Publication No. 57-212268, and an organic compound mixture solution made of an organic indium complex and a tin complex with coordination of acetylacetonate (for example, refer to Japanese Examined Patent Application Publication No. 63-25448, Japanese Examined Patent Application Publication No. 2-20706, and Japanese Examined Patent Application Publication No. 63-19046 are disclosed.
In most of these conventionally-known coating liquids, a nitrate of indium or tin, an organic or inorganic compound made of a halide, an organometallic compound such as a metal alkoxide, and others are used.
Since the coating liquid using a nitrate or a halide generates corrosive gas such as a nitrogen oxide or chlorine at the time of heating, there is a problem of causing corrosion of facilities and environmental pollution. As for the coating liquid using a metal alkoxide, the material is prone to hydrolytic degradation, thereby posing a problem in stability of the coating liquid. Moreover, most of the coating liquids using an organometallic compound described in the patent documents described above has poor wettability with respect to a substrate, and there is also a problem in which a non-uniform film tends to be formed.
To get around this, as a coating liquid with these problems mitigated, a coating liquid for forming transparent conductive film containing one or two types of solvent selected from a group of indium acetylacetonate (standard nomenclature: tris(acetylacetonato)indium: In(C5H7O2)3), tin acetylacetonate (standard nomenclature: di-n-butyl bis(2,4-pentanedionato) tin: [Sn(C4H9)2(C5H7O2)2]), hydroxypropylcellulose, alkylphenol, and alkenylphenol, and one or two types of solvent selected from dibasic acid ester and benzyl acetate is disclosed (for example, refer to Japanese Unexamined Patent Application Publication No. 6-203658.
In this coating liquid disclosed in Japanese Unexamined Patent Application Publication No. 6-203658, with hydroxypropylcellulose being contained in a mixture solution of indium acetylacetonate and tin acetylacetonate, wettability of the coating liquid with respect to the substrate is improved. At the same time, the viscosity of the coating liquid is adjusted based on the content of hydroxypropylcellulose, which is a viscosity-adjusting agent, thereby making it possible to adopt various coating methods such as spin coating, spray coating, dip coating, screen printing, and wire bar coating.
Also, as an improved coating liquid for spin coating, a coating liquid for forming transparent conductive film is suggested (for example, refer to Japanese Unexamined Patent Application Publication No. 6-325637) containing an organic indium compound such as indium acetylacetonate or indium octylate, an organic tin such as tin acetylacetonate or tin octylate, and an organic solvent, in which as the organic solvent, an acetylacetone solution with one or two types of solvent selected from a group of alkylphenol and alkenylphenol dissolved therein or the acetylacetone solution with one or two types of solvent selected from a group of alkylphenol and alkenylphenol dissolved therein being diluted with alcohol is used.
This coating liquid has a low viscosity, and can be used not only in spin coating but also spray coating and dip coating.
Meanwhile, since the ITO film contains indium, which is a rare metal and thus expensive, the material cost of the transparent conductive film itself is expensive. Moreover, since indium is a rare metal, fluctuations (inflation) in price of the indium metal disadvantageously tend to significantly fluctuate the material cost of the transparent conductive film. Therefore, When the transparent conductive film is applied to a transparent electrode for an element of a touch panel, a touch sensor, a liquid-crystal display (which may be referred to as an LCD), an electroluminescent display (which may be referred to as an ELD), electronic paper, an electrochromic display (which may be referred to as an ECD), or others, there is a limitation to aim at stable low cost.
Therefore, inexpensive transparent conductive films in place of the ITO film have been searched for, and a method of obtaining high transparency and conductivity equivalent to those of the ITO film by using a TiO2 film (a Nb:TiO2 film) doped with niobium has been reported (refer to Furubayashi, et al., Applied Physics Letter, vol. 86, 2005, p. 252101; and Proceedings of the 67th Meeting of the Japan Society of Applied Physics, autumn 2006, pp. 566 to 567, 30p-RA-12 to 16. The niobium-doped TiO2 film (NTO film) has low resistance that has been provided by performing a film formation on a glass substrate at a substrate temperature of 250° C. to 500° C. using a physical vapor phase growth method such as a sputtering method, a pulsed laser deposition method (a PLD method), or an electron beam deposition method (an EB deposition method), and then performing a heating process (reducing annealing) under a reducing atmosphere at 250° C. to 500° C.
As such, in place of the transparent conductive film having indium oxide as a main component, for example, a transparent conductive film having titanium oxide, which is a metal oxide other than indium oxide as described above, as a main component is desirable. Furthermore, a coating liquid for forming transparent conductive film capable of forming, at low cost, the transparent conductive film as described above also having excellent transparency and conductivity has been desired.
As to a metal oxide film, a wide variety of use application thereof, not limited to that of a transparent conductive film, has been considered. For example, a metal oxide film made of hafnium oxide (HfO2), zirconium oxide (ZrO2), or the like has attracted attention in recent years as a gate insulating film of a thin-film transistor element, and has been actively studied.
So far, as a gate insulating film of a field-effect thin-film transistor (TFT) element, silicon dioxide (SiO2), which forms a silicon oxide film, has been widely used in general. However, with rapid advance of microfabrication of elements, due to a low relative permittivity of silicon oxide (ε=3.9), making the gate insulating film thinner by microfabrication of elements has a limitation (an increase in gate leak current due to tunnel current). To address this problem, as described above, application of a metal oxide with high permittivity (hafnium oxide has a relative permittivity ε=approximately 30, and zirconium oxide has a relative permittivity ε=approximately 25) to a gate insulating film has been disclosed (refer to Takeshi Yamaguchi et al., “Study on Zr-Silicate Interfacial Layer of ZrO2-MIS Structure Fabricated by Pulsed Laser Ablation Deposition Method”, Extended Abstracts of the 2000 International Conference on Solid State Devices and Materials, Aug. 29 to 31, 2000, pp. 228 to 229.
To form such a metal oxide film with high permittivity, a vapor deposition method (a vapor phase method) such as Metal-Organic Chemical Vapor Deposition (MOCVD) or Atomic Layer Deposition (ALD) is generally used, and no coating method is used.
Also, it has been studied in recent years that a metal oxide film with a work function larger than that of an ITO film as a transparent conductive film (on the order of 4.6 eV to 4.8 eV) is applied to an organic electroluminescent element (also referred to as an organic EL element) as mentioned below.
Specifically, as referred to in Japanese Unexamined Patent Application Publication No. 9-063771, an attempt has been suggested in which any single or more of metal oxide films with a high work function such as ruthenium oxide (RuO2), vanadium oxide (VO2.5), and molybdenum oxide (MoO3) are laminated on the ITO film described above to facilitate injection of holes (positive holes) from the ITO film side as an anode electrode to a hole (positive hole) transport layer (in some cases, directly to a light emitting layer) and thereby achieve low-voltage driving, an improvement in luminous efficiency, and long life of the organic EL element. However, while Japanese Unexamined Patent Application Publication No. 9-063771 describes the use of a physical vapor phase growth method such as an electron beam deposition method, a direct current sputtering method, an RF magnetron sputtering method, or an ICB deposition method to form a metal oxide film with high work function, the use of a coating method is not described.
Furthermore, as metal oxide films, titanium oxide and cerium oxide can be used as ultraviolet ray cut coating; aluminum oxide (refractive index=1.62), silicon oxide (refractive index=1.46), titanium oxide (refractive index=2.5 to 2.7), zirconium oxide (refractive index=2.15), cerium oxide (refractive index=2.1 to 2.5), hafnium oxide (refractive index=1.91 to 2.15) and niobium oxide (refractive index=2.2 to 2.3) can be used for, by way of example, various optical coatings such as antireflective films and highly reflective films with application of their refractive indexes and transparency. Also, aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, and cerium oxide can be used for, by way of example, insulating overcoating formed on a (transparent) conductive film with application of their transparency and insulating property, and tungsten oxide can be used as an electrochromic display material or a near-infrared-ray absorbing material (a heat-ray shielding material) as a cesium-doped tungsten oxide doped with cesium or the like.
It has been difficult to obtain a metal oxide film with high quality and high permittivity suitable as a gate insulating film of a thin-film transistor element, a metal oxide film with high quality and high work function suitable as a hole (positive hole) injection layer of an organic EL element, or various metal oxides suitable also for other purposes by using a coating method at a relatively low baking temperature. That is, in a metal oxide film formed with processes of coating a substrate with a coating liquid for forming metal oxide film, drying, and heating, in the course of inversion of an organometallic compound and the like in that coating liquid to a metal oxide film by thermal decomposition and burning (oxidation) at the time of the heating process, the formed metal oxide fine particles are difficult to become dense, and therefore the obtained metal oxide film has low denseness, thereby posing problems such as a decrease in film strength and deterioration in film flatness. To address these problems, a metal oxide film with higher quality in view of aspects including film strength has been desired for use as a gate insulating film of a thin-film transistor element; a hole (positive hole) injection layer of an organic EL element; or a transparent electrode of a display, a touch panel, a solar cell, or the like.
In view of these circumstances, an object of the present invention is to provide a metal oxide film having high density and excellent in film strength formed by using an ink coating method, which is a low-cost and simple method of manufacturing a metal oxide film, a method of manufacturing the metal oxide film, an element using the metal oxide film, a substrate with the metal oxide film, and a device using the substrate with the metal oxide film.